Square Enix is paying tribute to the legendary David Bowie today, who unfortunately passed earlier this week. All you have to do is go onto the Square Enix store, add Omikron: The Nomad Soul to your cart, and use the promo code "omikron," at which point the $9.99 purchase will be discounted. Don't worry it's not proprietary -- you'll get a Steam code.
Bowie actually appeared in Omikron when the game was released back in 1999 as the in-game character Boz. He also reportedly had some creative control, and wrote a few songs specifically for the project.